[00:05.49]Why are some people left-handed?
[00:09.09]Scientists have been trying to answer that question for many years.
[00:14.32]Research has shown that ninety percent of people naturally use their right hands for most tasks.
[00:22.94]But hundreds of millions of people use their left hands.
[00:28.45]A study done in 1992 found that men are more likely to be left-handed than women.
[00:37.83]It also found that Asian or Hispanic people are less likely to be left-handed than white people,
[00:47.41]black people or North American Indians.
[00:51.33]Some cultures accept people who do things mostly with their left hands.
[00:58.10]Others do not.
[01:00.29]For example, in some countries eating with the left hand is considered an insult to others.
[01:08.81]In the past, some parents and teachers forced lefthanded children to use their right hands instead.
[01:17.69]Scientists want to know the reason for left-handedness
[01:23.67]because it is closely linked to mental problems like schizophrenia and language difficulties.
[01:30.91]One idea about the cause of left-handedness is the genetic theory.
[01:36.91]It says that people are right- or left-handed because of genes passed to them by their parents.
[01:44.62]For example, it has been shown that the handedness of adopted children
[01:49.87]is more likely to follow that of their birth parents than their adopted parents.
[01:55.93]Other evidence of genetic involvement can be found in some families.
[02:01.57]One famous example is the left-handed members of the present British Royal Family.
[02:08.84]These include the Queen Mother, her daughter Queen Elizabeth the Second,
[02:14.06]the Queen's son Prince Charles and his son Prince William.
[02:19.55]Amar Klar at the National Cancer Institute in Washington D.C. believes right-handed people are born with a gene for it.
[02:31.59]But, he says, about twenty percent of people do not have the right-handed gene.
[02:38.25]These people could be either left- or right-handed.
[02:43.13]Doctor Klar wants to find this gene.
[02:47.05]He plans to test the genes of members of 100 families
[02:53.17]in which at least one parent is right-handed and two of the children are left-handed.
[03:00.72]He will look for markers in the DNA to see how often the children have the same markers as the parents.
[03:10.41]Stanley Coren of the University of British Columbia in Canada thinks the reason for left-handedness is linked to fetal development.
[03:21.35]This idea says left-handedness results from damage to a fetus during or before birth.
[03:29.45]One cause of such damage could be an unusual position of the fetus in the mother's uterus.
[03:37.21]Another cause could be unusual levels of chemicals in the uterus.
[03:41.99]Doctor Coren says this damage could explain why a larger percentage of left-handed people have mental and health problems.
[03:52.86]Both these ideas have been offered to explain a situation that has been,a mystery to scientists for a long time:
[04:04.04]how can two babies who developed from the same egg and have the same genes use different hands?
[04:13.23]In 18% of identical twins,one twin is right-handec and the other is left-handed.
[04:21.93]One possible answer is that both twins lack the right-handed gene,
[04:28.33]so each has a chance to be either right-or left-handed.
[04:33.32]Another possible answer is that the difference is the result of damage caused by both babies being in a crowded uterus.
[04:43.90]Still other scientists say handedness could have more than one cause which could include both genetics and development.
[04:55.26]Experts also link left-handedness to artistic and athletic ability as well as extremely high intelligence.
[05:05.71]They say a great number of left-handed people were famous political leaders, artists, scientists or athletes.
[05:15.74]These include the political leaders Alexander the Great and Napoleon Bonaparte. The musicians Sergei Rachmanineff and Ludwig Van Beethoven.
[05:26.30]The artists Michelangelo,Leonardo da Vinci and Pablo Picasso.
[05:31.49]The scientist Albert Einstein.
[05:33.87]And the baseball player Babe Ruth.
[05:36.95]Other experts say every American president in the last twenty-five years has been at least partly left-handed,except for Jimmy Carter.
[05:48.50]They are Gerald Ford, Ronald Reagan,George Bush and Bill Clinton.
[05:54.87]However, some left-handed people were famous criminals.
[05:59.68]They include Billy the Kid, Jack the Ripper and the Boston Strangler.
[06:06.32]Much interesting information about left-handedness can be found on the Internet computer network. |